1. Field of the Invention
The present invention relates to an optical time-division multiplexer for use in optical transmission apparatus.
2. Description of the Related Art
An optical multiplexer module (OMUX module) having a plurality of optical paths with different optical path lengths provides a way to multiplex optical signals in a plurality of channels on the time axis (time-division multiplexing). Optical time-division multiplexer modules can be classified into two types, a fiber-delay type and a spatial-coupling type, according to the method by which the path length differences are created.
In the fiber-delay type of multiplexer module, the input light (consisting of short pulses occurring at fixed intervals) is generally branched in an optical coupler or splitter, and the branched pulses are modulated (by on/off data modulation) in an electro-absorption modulator. The modulated light pulses are launched into optical fibers of different lengths. After passing through the optical fibers, the light pulses are combined in a device such as a three-decibel (3-dB) coupler to obtain an optical signal in which a plurality of channels are multiplexed on the time axis.
The spatial-coupling type of multiplexer module employs optical elements such as half mirrors, rather than optical couplers or splitters and optical fibers, to create a plurality of optical paths on which light travels for different distances through space within the module.
The fiber-delay type of multiplexer module has a simple construction, but the refractive index of the cores of its optical fibers varies slightly with ambient temperature, stress, and torsion, shifting the phase of the light that has passed through the optical fibers. Moreover, a certain length of fiber must be allowed for interconnections between optical fibers, and between optical fibers and electro-absorption modulators, so the optical fibers in this type of module cannot be extremely short. As a result, the cumulative phase shift due to refractive index variation in the fiber cores is fairly large. Therefore, it has not been possible to maintain the optimal phase relationship, necessary for long-distance optical transmission, among the light pulses exiting the optical fibers; specifically, it has not been possible to reverse the phase of the multiplexed pulse light at each successive pulse (i.e., to shift the phase of the output light by π radians between successive pulses).
A spatial-coupling multiplexer module does not suffer from the problem of phase shift due to variation in the refractive index of the core of an optical fiber, but this type of multiplexer cannot compensate for changes in the wavelength of the input light. When the wavelength of the input light varies, the phase of the multiplexed output light also varies. Since the optical paths in a spatial-coupling multiplexer module are fixed, it has not been possible to change their lengths to compensate for these variations in the phase of the output light. A spatial-coupling multiplexer module employing waveguides (a planar lightwave circuit or PLC) has a similar disadvantage, and the further disadvantage of a large insertion loss.